Butler, K. H.: “Fluorescent Lamp Phosphors in Technology and Theory”, University of Pennsylvania, Press University Park (1980) discloses luminous bodies for fluorescent lamps which can be excited in both blue and short wavelength ultraviolet spectrum regions and emit light in a visible spectrum region. However, what has been known about these luminous bodies up to now is that they can be used only in fluorescent lamps and compact energy saving lamps.
Both organic dyes and inorganic dyes are used in LEDs. However, organic dyes, when used alone, are low in stability and are low in effectiveness.
WO 98/12757, WO 02/93116, and U.S. Pat. No. 6,252,254 disclose inorganic materials which will be more and more used as a color converting material for producing white light. They, however, should cope with a YAG luminous body system composed mainly of yttrium aluminum garnet. This luminous body species is disadvantageous in that the white light produced is on a low color rendering level, and, for example, in the case of a combination of a blue LED with this yellow light luminous body, color rendering index Ra is 70 to 77 which belongs to color rendering species IIa. Unsatisfactory quality of the produced white light is attributable to an elliptical light distribution which can provide only unsatisfactory white light even when a color temperature below 5000 K is provided. The use of the YAG luminous body system, however, is limited to use in blue LED. For a color wavelength of less than 460 nm, the excitation of the YAG luminous body system is dramatically decreased. In a color wavelength range of 370 to 400 nm in which UVLED is operated, the YAG luminous body system can be excited only to a limited extent.
WO 00/33389 describes a Ba2SiO4:Eu2+ luminous body system which has also good properties with stimulation in a color wavelength of less than 500 nm and exhibits maximum emission at a color wavelength of 505 nm.
WO 00/33390 discloses a luminous body mixture for LED which provides a color temperature of 3000 to 6500 K with a simultaneous color rendering index Ra of 83 to 87. Preferably, this luminous body, when used in combination with a further luminous body capable of emitting light in a red spectrum region or a blue spectrum region, can be used as a mixture for producing colored light and/or white light. All the conventional luminous bodies used in LEDs are disadvantageously unsatisfactory in temperature characteristics, as well as in thermostability. Due to these drawbacks, during the operation of LED, the effectiveness of the luminous body significantly decreases with increasing the temperature. As with the case of the YAG luminous body system, this causes a shift in energy distribution of light emission which causes a light color change.
For some of applications, conventional luminous bodies used in LEDs, for example, a Ce-activated YAG luminous body composed of barium magnesium aluminate and an Eu-activated BAM luminous body, have an additional drawback of a short fluorescence lifetime. The fluorescence lifetime of main conventional luminous bodies, that is, the Ce-activated luminous body and the Eu-activated luminous body, is typically a few microseconds. In some cases, the maximum fluorescence lifetime is a few milliseconds. This can be achieved, for example, by additional doping of manganese.
It is an object of the invention to provide a luminous body (or phosphor) that has an improved thermostability and a prolonged fluorescence lifetime associated therewith.
It is another object of the invention to provide an optical device that possesses excellent properties such as high light quality, good power saving, and high brightness.